2 cups freshly squeezed pink grapefruit juice (2 to 3 grapefruit)
1/2 cup elderflower liqueur, such as St-Germain
1/4 cup Campari
Simple syrup (see Cook's Note)
One 750-milliliter bottle sparkling wine, chilled
To make simple syrup, bring equal parts sugar and water to a boil in a small saucepan, stirring until the sugar dissolves. Cool and chill.